The aim of this research is the determination of the three-dimensional structure of the glycoprotein chloroperoxidase. This single enzyme uniquely exemplifies three groups of proteins: those which catalyze biological halogenation reactions, those which catalyze peroxide utilization, and those which contain covalently bound carbohydrate. Crystals have been grown which are adequate for detailed study by X-ray diffraction. It is anticipated that structural studies will provide a framework for critically testing the proposed enzyme mechanism and in so doing provide a model for biological halogenation. The three-dimensional structure of chloroperoxidase will also provide the data necessary for characterization of the interaction between protein and bound carbohydrate which might be generalized in understanding both how glycoproteins are synthesized and how protein and carbohydrate interact in glycoproteins.